Conventionally, a capacitance measurement apparatus that measures a capacitance value of a capacitive sensor such as a capacitor microphone whose electrostatic capacitance changes in response to received physical quantity (acceleration, pressure, gas, light, sound wave and so on) is known. FIG. 1 shows a conventional capacitance measurement apparatus 100. As shown in FIG. 1, the conventional capacitance measurement apparatus 100 includes an operational amplifier OP, an AC voltage generation apparatus OSC, a capacitive sensor Cs, resistance Rf that is feedback impedance. The AC voltage generation apparatus OSC generates an operation signal Vin that is applied to the capacitive sensor Cs at the time of measuring the capacitance. The capacitive sensor Cs and an inversion input terminal of the operational amplifier OP are connected by the signal line L. The resistance Rf is connected between the signal line L and the operational amplifier OP. Additionally, the capacitive sensor Cs is connected between the inversion input terminal of the operational amplifier OP and the AC voltage generation apparatus OSC. One terminal of the AC voltage generation apparatus OSC is connected to standard electric potential.
As for an operation of the conventional capacitance measurement apparatus 100 shown in FIG. 1, when voltage Vin from the AC voltage generation apparatus OSC is supplied, alternating current flows to the capacitive sensor Cs. In this case, since input impedance of the operational amplifier OP is ideally infinite, all the current that flows to the capacitive sensor Cs flows to the resistance Rf.
Output of the capacitance measurement apparatus Vout can be derived from the following method.
When the amplitude of the operation signal is V, the angular velocity of the operation signal is ωin, the standard capacitance of the capacitive sensor is Cd, the amplitude of change capacitance of the capacitive sensor Cs is C, and the angular velocity of capacitance change is ωc, the operation signal Vin and the capacitance of the capacitive sensor Cs can be represented byVin=V sin ωint  (1)Cs=Cd+C sin ωct  (2)
Since the current Is that flows through the capacitive sensor can be represented byIs=d(CsVin)/dt  (3)and the output Vout can be represented byVout=−IsRf  (4),by the expressions (1) through (4)Vout=−Rf{(Cd+C·sin ωct)·ωin·cos ωint+C·ωc·cos ωct·sin ωint}V  (5)is derived.
As is known from this expression (5), the output Vout has a term whose coefficient is the angular velocity of the capacitance change ωc. This means that in the case of the feedback impedance being the resistance, when the capacitance of the capacitive sensor changes at the frequency ωc, the output Vout that depends on the frequency ωc is outputted (the output Vout has frequency dependence). Consequently, in the case of the feedback impedance being the resistance, a processing circuit that does not have a frequency characteristic in the subsequent stage must be configured, and therefore there is a problem that the size of the circuit becomes large.
There, technology that the feedback impedance is configured not by the resistance but by the capacitance is proposed. FIG. 2 shows the capacitance measurement apparatus 101 whose feedback impedance is configured by the capacitance Cf. In this case, since the electric charge stored in the capacitive sensor Cs and that stored in the feedback capacitance Cf. are equal,−Cf·Vout=Cs·Vin  (6)holds, and therefore the output Vout can be represented byVout=−(Cd+C sin ωct)/Cf·V sin ωin  (7)
As is known from this expression, output voltage Vout does not include a term that is proportional to the angular velocity ωc. This is because the electric charge of the signal line L that is connected to two capacitances is maintained constantly when the feedback impedance is configured by the capacitance.
As described above, since the term that is proportional to frequency of capacitance change dose not appear in circuit output, there is no need to set up a processing circuit newly in the subsequent stage. As a result, it is possible to prevent the size of the circuit from becoming large.
However, in the case of configuring the feedback impedance by the capacitance Cf, the signal line L that connects Cf and the capacitive sensor becomes floating state electrically. For this reason, the electric potential of the signal line L becomes unstable and abnormality in a circuit operation that the circuit output is saturated with the power voltage may occur.
To prevent such a circuit abnormality, as shown in FIG. 2, it is conceivable to fix the electric potential of the signal line L by connecting resistance Rg between the signal line L and the ground.
However, in the case of fixing the electric potential by the resistance Rg, at the time of measuring the capacitance, there may be a case that potential difference in the both terminals of the resistance Rg is generated and that current flows through the resistance Rg. In that case, since the amount of electric charge varies in quantity, there is a problem that the sensibility of the capacitance measurement apparatus 101 decreases.
Consequently, it is desirable to propose a means to fix the electric potential of the signal line L without changing the electric charge quantity of the signal line L.
Additionally, when the standard capacitance Cd of the capacitive sensor Cs is very large compared with the capacitance change C, there is a problem that the capacitance change is not fully reflected in the output Vout.
Consequently, even if the standard capacitance Cd is very large compared with the capacitance change C, a circuit with satisfactory sensibility is desirable.
The present invention is done to solve the above-mentioned problems and the object of the present invention is to provide a capacitance measurement apparatus equipped with an electric potential fixing means for fixing the electric potential level of the signal line without changing the electric charge quantity of the signal line of the capacitance measurement apparatus and a standard capacitance cancel means for canceling the effect that the standard (fixed) capacitance of the capacitive sensor has on the circuit output.